The present invention relates to nuclear power plants and more particularly to fuel transfer systems employed therein during refueling to transfer fuel assemblies between the containment building and the storage pool.
When a nuclear reactor is shut down for refueling, fully or partially spent fuel assemblies are lifted from the reactor core in the containment building and moved through a pool of water to a fuel transfer system that transfers the assemblies usually one or two at a time to the auxiliary building where other apparatus takes the assemblies and deposits them in the pool storage area. New fuel assemblies or partially spent assemblies are carried by the fuel transfer system from the storage pool area to the containment building where they are placed in the reactor core.
Generally, the fuel transfer operation takes place under water to limit radiation exposure. Nonetheless, it is desirable that the fuel transfer system be economic in manufacture and both effective and reliable in operation to provide the required fuel assembly transport performance.
Typically, a stainless steel tube having a diameter between 20 inches and 36 inches provides a horizontal transfer path between the containment building and the spent fuel storage area. A transfer car may operate on a pair of spaced rails over the transfer path.
A basket is provided on the car to carry the fuel assembly during transfer. The basket may be an end pivot type in which case the basket is turned on its end to an upright position above the car for loading and unloading a fuel assembly at each end of the car travel. In this case, the space below the car and between the rails can be occupied with system structure since the basket does not pass through the undercar space when it is turned to the vertical position.
The basket may also be a center pivot type and this is normally the preferred scheme since the fuel assemblies can be upended at both ends of car travel with reduced loading on the upending mechanism. However, the center pivot basket does require undercar clearance space for the basket half that moves under the car when the basket is turned to the vertical position.
Since the car must operate in both directions along the transfer path, it is necessary that the drive system for the car provide driving force in either of the two directions. Further, it is desirable that the drive system be structured so that it is reliable for underwater operation.
One drive system architecture is the bilateral type and it involves placing a drive unit on the containment side of the containment wall to provide pulling force that directs the car away from the storage area and toward the containment building. Another drive unit located near the storage area provides pulling force that directs the car away from the containment building and toward the storage area. The fact that a drive unit must be located on opposite sides of the containment wall in this scheme is disadvantageous from a service and maintenance standpoint since a cable hookup must be provided from the drive unit to the car from the containment side before the fuel transfer operation can be started after a plant shutdown.
Another drive system architecture is the unilateral type and it involves apparatus advantageously located only on the storage area side of the containment wall. The drive system is organized so that it provides drive force for directing the car in either direction over the fuel transfer path.
In one prior art bilateral scheme, a fixed chain is welded to the bottom of the car midway between the rails and it is driven in either direction by a sprocket which in turn is driven by an underwater air motor.
Another prior bilateral scheme involves a continuous chain that runs along the center of the track. It is linked to the car and directly driven by a drive shaft of an underwater motor or indirectly driven through a coupling by a drive shaft of an electric motor above the water level. Again, the basket is end pivoted. A variation on this scheme involves a pair of continuous chains located outside the rails so that a center pivoted basket may be employed.
A prior unilateral scheme involves use of a fixed drum in the storage area and a drum on the car interconnected by cable. The car drum is coupled through a sprocket that engages pins on the rails to propel the car in one direction or the other. When the cable is pulled in one direction, the car drum is turned in one direction to propel the car in one direction along the track. When the cable is pulled in the opposite direction, the car is pulled in the opposite track direction and the sprocket rewinds the cable on the car drum. A center pivoted basket is used, but this scheme suffers from unreliability from a number of sources including the pin and sprocket drive arrangement.
In all of these schemes, underwater limit switches are normally required for system operation to enable the car to be brought to a controlled stop at its ends of travel. System reliability is accordingly adversely affected because the underwater limit switches are prone to leak over time.
The present invention is directed to a fuel transfer system having its drive system located on one side of the containment wall, preferably externally of the containment building. The drive is preferably structured for a center-pivoted car basket and otherwise for fuel transfer operation with significantly improved reliability.